Iced evaporator and low-pressure cycling in a walk-in freezer after defrost time was shortened
A 20 m³ walk-in freezer holding ice cream at -18 °C uses a 5 HP semi-hermetic R-404A condensing unit, electric defrost heaters, and a low-pressure switch set at 0.2 bar cut-out and 1.2 bar cut-in. Overnight the product rose to -8 °C and the evaporator is completely encased in ice; the fan blades are rubbing the ice. The condensing unit runs for about 90 s, then the LP switch cuts it out, and it restarts after a few minutes. Suction pressure falls to 0.2 bar during each run. The defrost timer motor is warm but the dial is hard to see; the owner says the defrost duration was reduced from 30 to 10 minutes last month to save energy. The three defrost heaters measure 42 Ω, 43 Ω, and 41 Ω. The defrost termination thermostat is open at -20 °C and closes when warmed in the hand. The drain line is clear. The store needs the freezer back tonight and asks you to raise the LP switch setting to stop the cycling.
1. Diagnose the iced evaporator and the LP cycling. Do the heater resistances and the termination thermostat prove the defrost system is healthy?
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The ice encases the evaporator and blocks airflow, so the coil absorbs very little heat. Suction pressure therefore falls quickly to the 0.2 bar cut-out, the compressor stops, pressure rises slowly, and the unit restarts for another short run. The LP cycling is a symptom of the iced coil, not the root cause. Raising the LP setting would only force the compressor to run longer against a blocked coil and could cause liquid floodback or compressor damage.
The heater readings of 42, 43 and 41 Ω show that the heater elements are continuous and roughly equal, but they do not prove that voltage is reaching them or that they draw current under load. The termination thermostat opening at -20 °C and closing when warmed in the hand is only a rough check; it must be tested against a thermometer at its specified cut-in and cut-out temperatures. The timer may also be stuck, may not be advancing into defrost, or may terminate too early. The most probable root cause is the shortened 10-minute defrost period, which is insufficient to clear the coil and allow the termination thermostat to end defrost properly.
2. What safe corrective plan would you follow to restore the freezer tonight, and what would you record after a successful defrost?
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Isolate the unit, switch off the fans, and defrost manually. Do not chip the ice with a sharp tool or pour water over electrical parts. Remove the ice safely, check that the drain is clear, and inspect the fan blades and evaporator fins for damage. Confirm the timer is set to the manufacturer's defrost duration, then test one complete defrost cycle while measuring heater current with a clamp meter, verifying that the termination thermostat ends defrost at the correct coil temperature, and confirming that the fans restart only after the coil temperature is suitable.
Record the defrost initiation and termination times, heater current for each element, termination thermostat cut-in and cut-out temperatures, drain condition, door seal condition, product temperature, and the LP switch settings. After the coil is clear, restore the original LP cut-out and cut-in; do not leave them altered to mask the fault. Monitor the first pull-down and the next automatic defrost before returning the freezer to normal service.